Multiwavelength lidar observation of a strange noctilucent cloud at Kuhlungsborn, Germany (54 degrees N)

On July 6/7, 1997, we observed a noctilucent cloud (NLC) by lidar at Kuhlun gsborn, Germany (54 degreesN, 12 degreesE) using four laser wavelengths (39 3, 423, 532, and 770 nm). While at the near-ultraviolet and visible wavelen gths clear backscatter signals were detected, the NLC did not produce signi ficant backscattering at the near-infrared wavelength 770 nm. The latter si gnature can not be explained by backscattering on any size distribution of homogeneous water ice spheres. In this work we discuss the spectral backsca tter signatures of particles with various shapes and compositions. We found three different particle types matching the experimental results, but ther e are physical arguments excluding these candidates as well. Our lidar obse rvations of this strange NLC event appear to contradict the common understa nding of NLC particle formation and sublimation. We emphasize the importanc e of multiwavelength observations for an in-depth interpretation of noctilu cent cloud lidar backscatter data, including also the infrared part of the spectrum.